1. Field of the Invention
The present invention relates to a wet friction material employed in friction engagement devices such as clutches and brakes used in oil in automatic transmissions of automobiles.
2. Related Background Art
An automatic transmission of an automobile, which is incorporated with a multiple-disc clutch comprising a plurality of friction plates formed by adhering a wet friction material on the surface a base plate (core plate) generally made of metals and separator plates as a friction counterpart made of a single sheet such as a metal sheet in an alternative manner, transmits or breaks a driving force by contacting these plates mutually under pressure or releasing the contact in an automatic transmission fluid (ATF) employed as a lubricant oil.
Paper wet friction materials called "paper friction materials" are generally employed as wet friction materials for friction engagement devices used in these oils. The wet friction materials are generally obtained by subjecting a fiber base material such as natural pulp fibers, organic synthetic fibers and inorganic fibers together with a filler such as diatomaceous earth as cashew resin and a friction controller to wet paper machining followed by impregnation of a resin binder comprising a thermosetting resin and heat hardening.
In the paper wet friction materials, thermosetting resins impregnated in machined paper material as resin binders not only bind a fiber base material and a filler, etc. and retain binding but also greatly affect friction properties and friction resistance of the wet friction materials. As resin binders, phenolic resins with excellent heat-resistance, high mechanical strength, and relatively good friction resistance are generally employed. The phenolic resins are produced by addition polymerization of phenols, such as phenol and cresol, and aldehydes, such as formaldehyde and paraformaldehyde, in the presence of an acid catalyst or alkaline catalyst. In the case of wet friction materials, resol-type phenolic resins obtained by addition polymerization in the presence of an alkaline catalyst are generally employed. As phenolic resins, unmodified phenolic resins have been employed practically in the past. They have such problems, however, that initial variations in friction coefficient are large in such a manner that a friction coefficient is low in an early phase due to local contact of a friction surface because they have a high cross-linking density and are thus hard, and a friction coefficient becomes higher with use due to increased conformability; and that burned spots called heat spots are generated due to a high temperature on a friction surface on a friction counterpart (separator plate) under high temperature and high load conditions, resulting of lack in long-term stability of friction properties.
In recent years, in order to improve these problems, modified phenolic resins have been extensively investigated and oil-modified phenolic resins and epoxy-modified phenolic resins, etc. with excellent flexibility have been studied and some of them have already been employed practically. These modified phenolic resins, however, are still inferior to unmodified phenolic resins in terms of heat resistance and friction resistance. Thus, satisfactory ones have not yet been attained.